1. Field of the Invention
The present invention relates to a dispersant for calcium carbonate. More particularly, the invention relates to a dispersant for calcium carbonate comprising a salt of an acrylic acid/maleic acid copolymer.
2. Description of the Prior Art
Precipitated calcium carbonate is mainly prepared by a process comprising calcining limestone together with anthracite coal or coke in a lime kiln to form unslaked lime, adding water to the resulting unslaked lime to form milk of lime and reacting the milk of lime with carbon dioxide gas. Various precipitated calcium carbonate products, differing in the particle size and shape, can be obtained by adjusting the reaction conditions in the above process. More specifically, the particle size can be changed in a relatively broad range of from 0.02-0.10.mu. to 1-3.mu. by adjusting the reaction conditions. Further, the particle shape can also be changed in the range of from a substantially cubic shape to a needle-like shape. Thus, various calcium carbonate products differing in the particle size and shape have been prepared by adjusting the reaction conditions.
When calcium carbonate is used in an aqueous medium, for example, for coating papers or for preparing aqueous paints, it is required that the calcium carbonate particles should have an excellent dispersibility. For example, when calcium carbonate is used for coating a paper, if the dispersibility thereof is poor, the viscosity and flowability of the coating suspension are degraded, a good coating cannot be otained and the printability of the resulting coated paper is very poor, resulting in the occurrence of various troubles. When a coated paper is prepared in this manner, water is removed after the coating is applied. Accordingly, the use of a high concentration pigment suspension having as low a water content as possible is desired. Therefore, a dispersant capable of providing a high concentration suspension having a low viscosity, when only a small amount of the dispersant is used, is eagerly desired in the art. As dispersants capable of meeting this requirement, there have been mainly used sodium pyrophosphate, sodium hexametaphosphate and sodium polyacrylate.
In Japan, finely divided precipitated calcium carbonate is mainly prepared by a process comprising introducing carbon dioxide gas into an aqueous suspension of calcium hydroxide to form calcium carbonate and precipitating the thus-formed calcium carbonate. The finely divided precipitated calcium carbonate prepared according to this process is recovered in the form of a slurry having a solid concentration of 15 to 25 wt. % and water is removed therefrom by a filter press so that the solid concentration is elevated to 40 to 60 wt. %. The resulting water-containing cake is marketed as it is or after it is dried and pulverized to form a powdery product. The above-mentioned cake having a solid concentration of 40 to 60 wt. %, which is obtained by dehydration using a filter press in the above-described process, is very hard and it is not flowable at all. Accordingly, the cake is rendered flowable by adding a dispersant, such as sodium pyrophosphate, sodium hexametaphosphate, sodium polyacrylate or the like. However, these dispersants are still insufficient and defective in that it is difficult to obtain a slurry having a sufficient flowability and a large quantity of the dispersant must be added so as to attain a sufficient flowability.